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MANUAL OF HISTOLOGY. 



laminae, longitudinally streaked and studded with nuclei and fusiform 

 cells, and a stratum of elastic membranes and fibrous networks arranged 

 longitudinally, between which the elements of smooth muscle may even 

 be insinuated. The middle tunic is formed of obliquely-crossed fibrous 



tissue, with elastic networks, 

 whose fibres take the same direc- 

 tion, and also of unstriped muscle 

 cells. Between these there ap- 

 pear, however, elastic membranes, 

 whose fibres maintain a longitu- 

 dinal course. The middle layer 

 of vessels of this kind is, as a 

 rule, much weaker than that of 

 arteries, but is rich in muscular 

 elements. The strong external 

 coat is formed of connective-tissue 

 with elastic interlacements. Un- 

 striped muscle may, however, also 

 occur. 



The largest veins of all, finally, 

 present a similar arrangement of 

 their internal laminae, except 

 that the latter have no unstriped 

 muscle fibres, while the middle 

 layer remains comparatively un- 

 developed, or may in rare cases 

 be entirely absent. The muscle 

 elements of the latter when pre- 

 sent are scanty, and accompanied 

 by abundant connective -tissue, 

 whose fibres are obliquely ar- 

 ranged. Elastic fibrous networks 

 of longitudinal direction are also 

 present still. A strange pecu- 

 liarity has been remarked here 

 in the usually strongly developed 

 external layer of many veins, 

 namely, the occurrence of a large 

 amount of longitudinally arranged 

 muscle, which generally occupies 

 the internal portion of the former 

 in varying strength : it is mixed 

 with fibres of connective- tissue 

 taking an oblique course. There 

 are certain veins, indeed, which 

 show an excessive development 

 of these muscular elements, as, 

 for instance, those of the preg- 

 nant uterus ; in others they are 

 entirely absent, as in the sinuses of the dura mater. 



The valves of veins, which are covered with endothelium, consist mainly 

 of connective-tissue interspersed with elastic fibres. 



In small arteries the internal and external layers remain comparatively 



Fig. 362. Transverse sections of arteries. 1. The 

 umbilical of a human foetus eight months old. a, 

 epithelium; 6, layers of the internal coat; c, the 

 muscular layers of the middle coat without any 

 intermixture of elastic elements; d, external cover- 

 ing, made up of colloid tissue. 2. A large artery 

 from the adult ; a and 6, as in fig. 1 ; c, the line of 

 demarcation between the inner and middle coats; 

 rf, elastic, and e, muscular laminae of the middle 

 coat; g, the external tunic traversed by elastic 

 networks; at/, below, the latter are highly deve- 

 loped. 



